1. Field
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred on (part of) the substrate (e.g., a glass plate) via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Instead of a circuit pattern, the patterning means can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate can be rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or which covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through the projection beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
In a conventional lithographic apparatus, the location of a substrate table (and hence the substrate which it supports) is measured with the aid of interferometers. Interferometers use a change in path length or phase of light reflected from a target to determine a change in distance.
It is known that the accuracy of results from an interferometer depend highly on the path the light takes. For example, it is necessary to take account of fluctuations in the air through which the light travels. These fluctuations include changes in temperature and pressure (or density) of the air. When the light used by the interferometer to measure changes in distance has to travel over a distance of, for example, about 300 mm, these fluctuations are easily minimized without compromising measurement accuracy, and even negligible in some circumstances. However, as the distance, or path length increases, and the error tolerance in substrate position decreases it becomes increasingly necessary, and correspondingly difficult, to accurately minimize these fluctuations.
There is currently a high consumer demand for larger flat panel displays, and this demand is predicted to continue and even increase in the coming years. The larger the flat panel display, the larger the substrate constituting that display needs to be. However, an increase in substrate size has associated disadvantages that need to be addressed.
Depending on how the lithographic apparatus applies the desired pattern onto the substrate, an increase in substrate size causes a corresponding increase in the path length of light used by the interferometer. The size of flat panel substrates has increased to such an extent that, for particular lithographic apparatus, the path length of light used by the interferometer can be of the order of about 6 meters (i.e., the distance from the interferometer to the point from which light is reflected (the beam length) can be of the order of 3 meters. The path length is defined as being twice the beam length). For such large path lengths, it is difficult to minimize the above-mentioned fluctuations, and consequently it is difficult or impossible to accurately position a substrate.
Therefore, what is needed is a system and method that can accurately determine location of and position a substrate regardless of a size of the substrate.